Method and apparatus for mapping the sensitivity of the face of a photodetector specifically a pmt

ABSTRACT

Apparatus and technique for obtaining color-coded area sensitivity maps of photomultiplier tubes. First and second oscilloscopes are driven in synchronism to produce rasters on their faces. The raster of the first oscilloscope is optically imaged on the face of the photomultiplier tube being checked. The output of the photomultiplier is applied to a detector which produces an output if and only if the output of the photomultiplier is between two voltages set into the detector. The output of the detector intensity modulates the second oscilloscope and a color camera takes a picture of the face of the second oscilloscope through a color filter. Different voltages are set into the detector and the camera takes another picture through a different color filter. This procedure is continued which results in a color-coded area sensitivity map of the photomultiplier tube. In an alternate embodiment of the invention a color television is used in place of the second oscilloscope, the camera and the color filter. In this embodiment a different detector is needed for each color.

United States Patent Youngbluth, J r.

[ Mar. 14, 1972 [54] METHOD AND APPARATUS FOR MAPPING THE SENSITIVITY OFTHE FACE OF A PHOTODETECTOR SPECIFICALLY A PMT [72] Inventor: OttoYoungbluth, Jr., Yorktown, Va.

[73] Assignee: The United States of America as represented by theAdministrator of the National Aeronautics and Space Administration [22]Filed; Manll, 1970 [2i] Appl.No.: 18,427

2,597,383 5/1952 Samuel ..324/20 CR 2,765,440 10/ l 956 Adelman .324/20R OTHER PUBLICATIONS lBM Technical Disclosure Bulletin, Determining Endof Nov. 1966 Primary Examiner-Rudolph V. Rolinec Assistant Examiner-R.J. Corcoran Attorney-Howard J. Osborn, William H. King and G. T. McCoy[5 7] ABSTRACT Apparatus and technique for obtaining color-coded areasensitivity maps of photomultiplier tubes. First and secondoscilloscopes are driven in synchronism to produce rasters on theirfaces. The raster of the first oscilloscope is optically imaged on theface of the photomultiplier tube being checked. The output of thephotomultiplier is applied to a detector which produces an output if andonly if the output of the photomultiplier is between two voltages setinto the detector. The output of the detector intensity modulates thesecond oscilloscope and a color camera takes a picture of the face ofthe second oscilloscope through a color filter. Different voltages areset into the detector and the camera takes another picture through adifferent color filter. This procedure is continued which results in acolor-coded area sensitivity map of the photomultiplier tube.

In an alternate embodiment of the invention a color television is usedin place of the second oscilloscope, the camera and the color filter. Inthis embodiment a difi'erent detector is needed for each color.

Phosphor Life in CRT s by Gaebelein et al., Vol. 9, No. 6, 7 Claims, 3Drawing Figures w f "/\/\/\/\O FUNCTION QHASE FUNCTION 7 STAIR STEP 7GENERATOR LOCK GENERATOR O @1 1?! GENERATOR \IY le 1 A O l u 29 T25" 6 LPHOTOMULTIPLIER CAMERA d SCAN |4 DISPLAY OSCILLOSCOPE 28 OSCILLOSCOPEHIGH LEVEL DETECTORO VOLTMETER i Hl REFERENCE VOL AGE iLO LOW LEVELDETECTOR PATENTEBHAR 1 4 i912 SHEET 2 BF 3 --N mq- IO up l -N md'mwh a,

INVENTOR. OTTO YOUNGBLUTH, JR.

PATENTEDMAR 14 m2 SHEET 3 OF 3 0 M m m m UTH INVENTOR. OTTO YOUNGBLUTH,JR.

MM /-4 ff ATTOR EYS METHOD AND APPARATUS FOR MAPPING THE SENSITIVITY OFTHE FACE OF A PHOTODETECTOR SPECIFICALLY A PMT ORIGIN OF THE INVENTIONThe invention described herein was made by an employee of the UnitedStates and may be manufactured and used by or for the Government forgovernmental purposes without the payment of any royalties thereon ortherefor.

BACKGROUND OF THE INVENTION The invention relates generally to measuringthe area sensitivity of photomultiplier tubes (PMTs) and morespecifically concerns apparatus and technique for obtaining colorcodedarea sensitivity maps of PMTs.

For many spaceor earth-oriented experiments that require lightdetection, researchers depend upon their most sensitive detectorthe PMT.Unfortunately, the PMT has certain undesirable characteristics. One ofthese is a nonuniform area sensitivity (that is, the output at the anodevaries with the perunit-illuminated area of the photocathode). Inapplication such as spectroscopy, radiometry, tracking, and so forth,where light is imaged on the face of the photomultiplier, the anodeoutput can vary as much as 50 percent or more with the location of theimage on the detector face. In this case, the greatest portion ofuniform area sensitivity of the PMT should be alined with the expecteddirection of the image motion. In the scintillation field, a preferredarea of the PMT face must be selected for placement of the scintillationcrystal to insure uniform sensitivity. This preferred area can belocated from a map of the PMT area sensitivity. Obtaining such a map isa tedious and time-consuming task; hence, the usual alternative is toassume the area sensitivity to be constant, or to average thenonuniforrnities with optics. The area sensitivity includes thenonuniformity of the photocathode, the collection efficiency at dynodeone and at the anode, and the effects of the electron multiplier. In aPMT with a focus electrode, the user has some control of the areasensitivity. However, this is usually set in accordance with themanufacturers recommendation. These recommendations vary with thespecific PMT and operation and include the following: (1) connect thefocus electrode to the cathode, (2) connect the focus electrode to thefirst dynode, and (3) adjust the focus electrode to obtain maximum anodecurrent. Unfortunately, the recommendations do not include dataconcerning the variation of the area sensitivity under these conditions.

SUMMARY OF THE INVENTION This invention provides means for obtainingcolor-coded area sensitivity maps for PMTs. The mapping technique uses araster of a scan oscilloscope to scan and illuminate the photocathode.The PMT output is applied to a window voltage detector, and the selectedwindow voltage modulates the brightness of a phase-locked raster of adisplay oscilloscope. A series of interchangeable color filters (one foreach selected window voltage) are used in color coding the photographsof the display raster. The resulting colors, normally six, are used torepresent the relative PMT output sensitivity for photons incident onvarious portions of the photocathode. Each color represents a constantanode output similar to isoelevation lines on a topographical map. Thistype of presentation, which takes less than minutes, provides theexperimenter with quantitative and qualitative information of thesensitivity variations across the sensitive surface of the detector.

The data obtained are useful in analyzing the PMT spectral responsecharacteristics, and in measuring the fatigue and circuit design effectson PMT area sensitivity. Using these data, optical and electricalparameters can be adjusted to tailor the PMT output characteristics forthe particular measurement problem. Representative photomultiplier dataobtained with this method show that the per-unit area sensitivity acrossthe tube face can vary as much as 50 percent, and that the bestuniformity is obtained by adjusting the focus electrode voltage to alevel between the photocathode and dynode one.

In an alternate embodiment of the invention a color television is usedin place of the display oscilloscope, the color filter and the colorcamera. With the use of a color television display, a full color picturecan be obtained in one raster (one frame); thus, only a single exposureis made instead of the multiple exposure of the film in the otherembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of apreferred embodiment of the invention;

FIG. 2 is a schematic drawing of the highand low-level detectors, andthe reference voltage source in FIG. I; and

FIG. 3 is a block diagram disclosing an alternate embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION Turning now to an embodiment ofthe invention selected for illustration in the drawings the number 11 inFIG. 1 designates the photomultiplier whose sensitivity is to be mapped.An oscilloscope 12 is driven to obtain a raster on the face of itscathode-ray tube (CRT). This raster is imaged on the face ofphotomultiplier 11 by a lens 14 through a blue filter 13. Blue filter 13is used to minimize the persistence effect. The y-axis of oscilloscopel2 and the y-axis of an oscilloscope 15 are driven by a IS-hertzstairstep waveform generator 16. The xaxis of oscilloscope 12 is drivenby a IS-hertz triangular wave from a function generator 17 and thex-axis of oscilloscope 15 is driven by a IS-hertz triangular wave fromfunction generator 18. These two triangular wave signals are identicalexcept that the one generated by function generator 18 is adjusted witha phase lock unit to compensate for phase shifting. Most of the phaseshift at frequencies in the l5-hertz range can be eliminated by using ashort decaY phosphor with oscilloscope 12.

The output of photomultiplier 11 is amplified by DC amplifiers l9 and20, and filtered by a capacitor 21. The output of amplifier 20 isapplied to a high-level detector 22 and a lowlevel detector 23. Theoutput of amplifier 20 is also integrated by an integrator 24 and theresulting voltage is measured by a voltmeter 25. Detectors 22 and 23have different reference voltages applied to them from the referencevoltage source 26. The reference voltage applied to detector 22 ishigher than the reference voltage applied to detector 23. These twodetectors are identical and each will produce one voltage if its inputvoltage is higher than its reference voltage and will produce anothervoltage if its input voltage is lower than its reference voltage. Theoutputs of detectors 22 and 23 are applied to a difference amplifier 27whose output is applied to the z-axis of oscilloscope 15 to intensitymodulate the raster pattern. There is a voltage applied to the z-axis ofoscilloscope 15 only while the voltage at the output of amplifier 20 isat a level between the reference voltage applied to detector 22 and thereference voltage applied to detector 23. If the voltage at the outputof amplifier 20 is higher or lower than both reference voltages novoltage is applied to the z-axis of oscilloscope 15 since the outputs ofthe two detectors are the same and are cancelled by difference amplifier27.

The time that difference amplifier 27 produces a voltage represents thearea of the face of photomultiplier 11 that has a sensitivity greaterthan that corresponding to the reference voltage applied to detector 23and less than that corresponding to the reference voltage applied todetector 22. The resulting lighted area on oscilloscope 15 representsthe area on the face of photomultiplier having a sensitivity in therange defined by the two reference voltages. This lighted area isphotographed through a color filter 28 by a multiexposure color camera29. For each set of reference voltages applied to detectors 22 and 23 adifferent colored filter 28 is used. Hence, camera 29 produces amulticolored picture with the area of each color representing the areaof the face of photomultiplier II having a sensitivity corresponding tothat color.

In FIG. 2 there is shown a schematic diagram suitable for use ashigh-level detector 22, low-level detector 23 and reference voltagesource 26. High-level detector 22 consists of transistors 31 and 32, andtheir associated circuitry. The output from amplifier 20 is appliedthrough terminal 33 and resistor 34 to the base of transistor 31, andthe high reference voltage from source 26 is applied through a resistor35 to the base of transistor 32. If the voltage at terminal 33 isgreater than the high reference voltage then transistor 31 is conductiveand transistor 32 is nonconductive causing a first voltage level to beproduced at terminal 36; and if the voltage at terminal 33 is less thanthe low reference voltage then transistor 31 is nonconductive andtransistor 32 is conductive causing a second voltage level to beproduced at terminal 36. Detector 23 is identical to detector 22 exceptthat the low reference voltage is applied through a resistor 37 to thebase of a transistor 38. The output of amplifier 20 is applied through aresistor 39 to the base of a transistor 40. The output of detector 23 isterminal 41. Terminals 36 and 41 are connected to difference amplifier27. Reference voltage source 26 consists of a reference voltage appliedto terminal 42, a bank of resistors 43 connected in series and to thereference voltage, and a switch 44 for selecting one of the resistors inbank 43 to connect it between transistors 32 and 38. For each setting ofswitch 44 a voltage difference at a different level is selected. Analternative reference voltage source 26 is switch 45 in its otherposition so that reference voltage 42 is connected across potentiometer46 with switch 44 connected to the 7 terminals.

In the operation of this invention Oscilloscopes l2 and 15 are turned onand they are driven by function generators l6, l7 and 18 to obtain araster on the face of each CRT. Reference voltage source is then set toapply selected voltages to detectors 22 and 23. This can be done byplacing the movable contacts of switch 44 on fixed contacts 1. Then afilter 28 having a color selected to represent the level of the voltagesapplied to the detectors is placed between the CRT of oscilloscope l andcamera 29 which then takes the picture. Inasmuch as the only area on theface of the CRT that is lighted is that area corresponding to the areaon the face of photomultiplier 11 that has a sensitivity represented bythe difference in voltages applied to the detectors, the picture takenby the camera is only that area. This procedure is continued fordifferent settings of the reference voltage source 26 and correspondingdifferent color filters 28. The resulting composite picture taken bycamera 29 is a multicolored picture with the area of each color denotinga specific sensitivity for the corresponding area on the face ofphotomultiplier l 1 In the alternate embodiment of the inventiondisclosed in FIG. 3, a color television 50 is used in place of displayoscilloscope in F 1G. 1. That is, the x-axis (terminal 51) of colortelevision 50 is connected to function generator 18 and the yaxis(terminal 52) is connected to stairstep generator 16. Each of thedetectors 53 includes a high-level detector 22, a lowlevel detector 23and a reference voltage source 26. A different detector 53 is used foreach color that is displayed on the screen of color television 50.Terminal 36 of high-level detector 22 and terminal 47 of low-leveldetector 23 in each detector 53 are applied to an AND-gate 54. Signalconditioner 55 changes the polarity of signal at the output of AND-gate54, improves the shape of this signal, provides gain control, andprovides individual color intensity control. The outputs from all signalconditions 55 are applied to a diode matrix 56 having three outputs: oneoutput supplies the blue gun and the other output supplies the greengun. Hence, the combination of the color guns of television 50 that areactivated at any given time depends on the signal conditioners 55 thatis producing a signal at that time. AND gates, signal conditioners anddiode matrices are well known devices and hence are not disclosed indetail in this application.

In the operation of the alternate embodiment of the invention a rasteris produced on the faces of scan oscilloscope 12 and color television50. The raster on oscilloscope 12 is imaged on photomultiplier 11 andthe resulting output from amplifier 20 is applied to all detectors 53.Each detector 53 has high and low reference voltages applied to it. Asthe raster scans across the face of photomultiplier 11 the output ofamplifier 20 changes due to the nonuniform sensitivity ofphotomultiplier 11. If at any given time the voltage at the output ofamplifier 20 is between the high and low reference voltages of detector53 the associated AND-gate 54 produces a signal which is conditioned bythe corresponding signal conditioner 55 and then applied to diode matrix53. This signal applied to diode matrix 53 will activate one or more ofthe color guns of televisions 50 to produce the color on its screenassociated with the interval between the high and low referencevoltages. Hence, the raster on photomultiplier 11 produces a color-codedcontour map on color television 50 that is indicative of the sensitivityof the face of the photomultiplier 11.

The advantages of this invention is that it provides color contour mapsso that an experiment can study various effects upon the areasensitivity of PMTs. These include such effects as fatigue, wavelength,focus electrode configuration and voltage, compensating transmissionfilters, resistor divider net works, diffusing glasses, and so forth.With the capability of readily determining the PMT area sensitivity andhow it can be varied, the experimenter can use these devices in a mannerthat has been previously impractical.

It is to be understood that the form of the invention herewith shown anddescribed is to be taken as a preferred embodiment. Although thepreferred embodiment only includes maps of PMTs, it can be easilymodified to obtain maps of other photodetectors.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. Apparatus for mapping the sensitivity of the face of a photodetectorcomprising: first and second oscilloscopes; means for driving said firstand second Oscilloscopes to obtain synchronized rasters on their faces;means for optically imaging the raster on the face of said firstoscilloscope on the face of said photodetector; detector means, havingfirst and second voltages set into it, connected to the output of saidphotodetector for producing a voltage if, and only if, the output ofsaid photodetector after amplification is between said first and secondvoltages; and means for connecting the output of said detector means tosaid second oscilloscope to intensity modulate the raster on its facewhereby the modulated area of the raster on the face of said secondoscilloscope corresponds to the area on the face of said photodetectorhaving a sensitivity determined by said first and second voltages.

2. Apparatus according to claim 1 including a color camera for taking apicture of the face of said second oscilloscope and a colored filterlocated between said camera and the face of said second oscilloscopewhereby the picture taken by said camera is a colored area correspondingto the area of sensitivity on the face of said photodetector representedby the color of said colored filter.

3. Apparatus according to claim 2 wherein said detector means includesmeans for changing said first and second voltages, said colored filteris interchangeable with other colored filters, and said camera is amultiexposure camera whereby a color-coded contour sensitivity mappingof the face of said photodetector can be made.

4. Apparatus according to claim 1 wherein said detector means includes afirst circuit connected to said first voltage for producing a thirdvoltage while the output of said photodetec tor exceeds said firstvoltage and for producing a fourth voltage while said first voltageexceeds the output of said photodetector; a second circuit connected tosaid second voltage for producing said third voltage while the output ofsaid photodetector exceeds said second voltage and for producing saidfourth voltage while said second voltage exceeds the output of saidphotodetector; and a difference amplifier connected to the outputs ofsaid first and second circuits whereby said difference amplifierproduces an output if and only if the output of said photodetector is ata level between said first and second voltages.

5. Apparatus for mapping the sensitivity of the face of a photodetectorcomprising: an oscilloscope; a color television; means for driving saidoscilloscope and said color television to obtain synchronized rasters ontheir faces; means for optically imaging the raster on the face of saidoscilloscope on the face of said photodetector; a plurality of detectormeans with each having first and second voltages set into it andconnected to the output of said photodetector for producing a voltage ifand only if the output of said photodetector after amplification isbetween said first and second voltages; and means for connecting theoutputs from said plurality of detector means to said color televisionto produce a different color for each detector means that produces anoutput signal whereby said color television produces a color-codedmapping of the sensitivity of the face of said photodetector.

6. Method for mapping the sensitivity of the face of a photodetectorcomprising the steps of: driving first and second Oscilloscopes toobtain synchronized rasters on their faces; optically imaging the rasteron the face of said first oscilloscope on the face of saidphotodetector; producing a voltage if and only if the resulting outputof said photodetector is between two preselected values; intensitymodulating the raster on the face of second oscilloscope with saidproduced voltage; and taking a picture of the face of said secondoscilloscope through a colored filter whereby the picture taken is acolored area corresponding to the area of sensitivity on the face ofsaid photodetector represented by the color of said colored filter.

7. Method according to claim 6 including the additional step of changingsaid two preselected values and the color of said colored filter toobtain a multicolored picture.

1. Apparatus for mapping the sensitivity of the face of a photodetectorcomprising: first and second oscilloscopes; means for driving said firstand second oscilloscopes to obtain synchronized rasters on their faces;means for optically imaging the raster on the face of said firstoscilloscope on the face of said photodetector; detector means, havingfirst and second voltages set into it, connected to the output of saidphotodetector for producing a voltage if, and only if, the output ofsaid photodetector after amplification is between said first and secondvoltages; and means for connecting the output of said detector means tosaid second oscilloscope to intensity modulate the raster on its facewhereby the modulated area of the raster on the face of said secondoscilloscope corresponds to the area on the face of said photodetectorhaving a sensitivity determined by said first and second voltages. 2.Apparatus according to claim 1 including a color camera for taking apicture of the face of said second oscilloscope and a colored filterlocated between said camera and the face of said second oscilloscopewhereby the picture taken by said camera is a colored area correspondingto the area of sensitivity on the face of said photodetector representedby the color of said colored filter.
 3. Apparatus according to claim 2wherein said detector means includes means for changing said first andsecond voltages, said colored filter is interchangeable with othercolored filters, and said camera is a multiexposure camera whereby acolor-coded contour sensitivity mapping of the face of saidphotodetector can be made.
 4. Apparatus according to claim 1 whereinsaid detector means includes a first circuit connected to said firstvoltage for producing a third voltage while the output of saidphotodetector exceeds said first voltage and for producing a fourthvoltage while said first voltage exceeds the output of saidphotodetector; a second circuit connected to said second voltage forproducing said third voltage while the output of said photodetectorexceeds said second voltage and for producing said fourth voltage whilesaid second voltage exceeds the output of said photodetector; and adifference amplifier connected to the outputs of said first and secondcircuits whereby said difference amplifier produces an output if andonly if the output of said photodetector is at a level between saidfirst and second voltages.
 5. Apparatus for mapping the sensitivity ofthe face of a photodetector comprising: an oscilloscope; a colortelevision; means for driving said oscilloscope and said colortelevision to obtain synchronized rasters on their faces; means foroptically imaging the raster on the face of said oscilloscope on theface of said photodetector; a plurality of detector means with eachhaving first and second voltages set into it and connected to the outputof said photodetector for producing a voltage if and only if the outputof said photodetector after amplification is between said first andsecond voltages; and means for connecting the outputs from saidplurality of detector means to said color television to produce adifferent color for each detector means that produces an output signalwhereby said color television produces a color-coded mapping of thesensitivity of the face of said photodetector.
 6. Method for mapping thesensitivity of the face of a photodetector comprising the steps of:driving first and second oscilloscopes to obtain synchronized rasters ontheir faces; optically imaging the raster on the face of said firstoscilloscope oN the face of said photodetector; producing a voltage ifand only if the resulting output of said photodetector is between twopreselected values; intensity modulating the raster on the face ofsecond oscilloscope with said produced voltage; and taking a picture ofthe face of said second oscilloscope through a colored filter wherebythe picture taken is a colored area corresponding to the area ofsensitivity on the face of said photodetector represented by the colorof said colored filter.
 7. Method according to claim 6 including theadditional step of changing said two preselected values and the color ofsaid colored filter to obtain a multicolored picture.